1. Field of the Invention
This invention relates to a method for the recovery of primarily gold and silver from low-grade refractory ores whose refractory nature is caused by their content of sulfidic and/or organic carbonaceous matter, and more particularly, it relates to an agglomerative heap leaching method for recovering gold and silver from such ores.
2. Prior Art
In recent years the price of gold has increased dramatically. At the same time, many of the existing high-grade, free milling gold ore deposits have become largely exhausted after many years of active mining. Due to the gold price increase and to the increasing scarcity of high grade, free milling ore deposits, the mining industry has intensified its efforts to develop techniques for processing low grade gold ores, i.e. ores containing less than about 0.10 ounces gold per ton of ore. Large deposits of such gold ores occur in various countries throughout the world, including the United States. Similarly, such large deposits of low grade silver ores occur throughout the world.
Many of these low grade gold and silver deposits present a particularly difficult problem to would-be processors because of the refractory nature of the ore, i.e. refractory ores are ores whose gold and silver values cannot be economically recovered by conventional cyanidation techniques. The refractory nature of an ore can be caused by a number of factors. For example, certain sulfidic and/or organic carbonaceous components, if contained in an ore, are especially notorious for causing the ore to be resistant to conventional cyanidation. The present process is directed toward those ores whose refractory nature is largely caused by sulfidic and/or carbonaceous matter.
The mining industry has developed a number of successful processes for treating high grade gold ores which are refractory because of their content of sulfidic and/or organic carbonaceous matter. Two of the best known processes, described in U.S. Pat. No. 4,289,532 to Matson et al. and U.S. Pat. No. 4,038,362 to Guay, subject the refractory ores to a preoxidation treatment step utilizing chlorine and chlorine--containing compounds. Although the preoxidation treatments are very effective in rendering the ores nonrefractory, the exact method by which the preoxidation treatment works is not yet completely understood. In some instances, the refractory components are converted to nonrefractory components, e.g. refractory sulfides are converted to nonrefractory sulfates. In other instances, refractory organic carbonaceous components are deactivated so that they do not compete with activated carbon and adsorb gold in subsequent cyanide leach/carbon adsorption operations. In still other instances, the refractory components are destroyed, decomposed or in some other manner, often unexpected, rendered nonrefractory by the preoxidation step. These preoxidation treatment processes are too expensive to economically treat the low grade refractory ores which are the subject of the present invention.
Heap leaching has long been used to recover gold, silver and other valuable minerals from ores where either the ore grade or the amount of reserves is inadequate to justify a conventional plant. In recent years, its use in the gold mining industry has accelerated as the demand and price of gold have risen. One of the most attractive features of heap leaching is its simplicity. Ore is placed on an impervious base and a lixiviant for gold and silver, usually an alkaline cyanide solution, is sprinkled over the ore and allowed to percolate through the heap. The desired metal-bearing solutions drain from the heaps, are collected in ponds, and the desired metal values are recovered in activated carbon columns, in zinc precipitation units or by other means. Heap leaching of ores with good percolation characteristics will result in metal recovery efficiencies of over 50%.
If an ore has an excessively high clay or fines content as is often the case, agglomeration or binding of the fine ore particles prior to heap leaching is usually recommended. The agglomeration procedure consists of mixing the crushed ore with portland cement and/or lime and water which may contain cyanide, tumbling the mixture to allow agglomerates to form, and then aging or curing the product for a period of time. Agglomeration increases solution flow rates, improves recovery efficiency and reduces the leaching cycle time. Often the improvements are dramatic and, as a result, the use of agglomerative heap leaching to recover gold from low grade ores is rapidly growing. Unfortunately, low-grade ores which are also refractory because of their content of sulfidic and/or organic matter will often continue to exhibit relatively poor recovery efficiencies, i.e. less than about 40% when agglomerative heap leaching methods, as presently practiced, are used.
A need thus exists, and particularly in the gold and silver mining industry, for an agglomerative heap leaching method that can economically recover gold and silver from low grade ores which are refractory because of their content of sulfidic and/or organic carbonaceous matter.
The object of this invention is to provide an economic agglomerative heap leaching method for recovering gold and silver from low grade ores which are refractory because of their content of sulfidic and/or organic carbonaceous matter.